Direct sorbent preparation/feed apparatus and method for circulating fluidized bed boiler systems

ABSTRACT

A sorbent conditioning and feed apparatus for a circulating fluidized bed combustion system includes a dryer apparatus configured to dry unprocessed sorbent material transported thereto from a raw storage container; a sorbent crushing device configured to reduce the particle size of dried sorbent material discharged from the dryer apparatus; and a circulating fluidized bed (CFB) boiler configured to receive processed sorbent material conveyed from the sorbent crushing device; wherein the dryer apparatus utilizes an untempered hot air source diverted directly from a primary air stream input to the circulating fluidized bed boiler.

TECHNICAL FIELD

The present disclosure relates generally to circulating fluidized bed(CFB) combustion systems and, more particularly, to an improved directsorbent preparation/feed apparatus and method for a CFB boiler system.

BACKGROUND

Fluidized bed combustion (FBC) is a combustion technology used in powerplants primarily to burn solid fuels. FBC plants are more flexible thanconventional plants in that they can be fired on coal, coal waste orbiomass, among other fuels. The term FBC covers a range of fluidized bedprocesses which include Circulating Fluidized Bed (CFB) boilers,Bubbling Fluidized Bed (BFB) boilers and other variants. Fluidized bedssuspend solid fuels on upward-blowing jets of air during the combustionprocess, resulting in a turbulent mixing of gas and solids. The tumblingaction, much like a bubbling fluid, provides a means for more effectivechemical reactions and heat transfer.

During the combustion of fuels that have a sulfur containingconstitutent, coal for example, sulfur is oxidized to form primarilygasous SO₂. In particular, FBC reduces the amount of sulfur emitted inthe form of SO₂ by a desulfurization process. A suitable sorbent, suchas limestone containing CaCO₃, for example, is used to absorb SO₂ fromthe flue gas during combustion. In order to promote both combustion ofthe fuel and the capture of sulfur, FBC combustion operates attemperatures lower than conventional combustion systems. FBC systemsoperate in a range typically between about 780° C. and about 1000° C.Since this allows coal to combust at cooler temperatures, NO_(x)production during combustion is lower than other coal combustionprocesses. Fluidized-bed boilers evolved from efforts to find acombustion process able to control pollutant emissions without externalemission controls (such as scrubbers).

CFB boiler systems are generally associated with limestone feed systemsfor sulfur capture. Processed limestone fed to a boiler is typicallyconditioned by means of size reduction machines to specific size rangesto allow for the desulfurization process to proceed efficiently. If theparticles are too large, the desulfurization process will not beefficient because there is insufficient limestone particle surface areato react with the flue gas. On the other hand, if the particles are toosmall, the limestone will be carried out of the desulfurization zonewith the flue gas before it can react to remove the sulfur. Typically,limestone is fed to the boiler with a median particle diameter in therange of (as an example, but not limited to) about 100 to about 400microns. In order to achieve this particle size range, unprocessed, rawlimestone is reduced in both size and moisture content by size reducingmachines. Presently, there are various machines available for crushinglimestone, including for example, hammer mills, roll crushers and rollermills. Regardless of the type of equipment used for limestone crushing,the particles are dried either before or during crushing in order toproduce a freely flowing material.

Traditionally, limestone is prepared separately from the boiler system,either on-site or by the limestone supplier. Prepared limestone isconveyed to a storage system in the boiler house from which it isthereafter metered and injected into the boiler. Experience has shownthat the cost of prepared limestone using separate on-site systems orsupplied from off-site vendors is expensive. In the case of separate,on-site systems a separate building and auxiliary fuel is used to drythe limestone. On the other hand, a limestone preparation and feedsystem may also be integrated with the boiler system itself, resultingin a significant reduction in capital and operating costs. Specifically,CFB boilers may be equipped with an integrated limestone preparation andfeed system that resides in the boiler building. Such a system thatdries and prepares limestone as needed is also referred to aJust-In-Time (JIT) limestone system.

Systems using roller mills have also been designed and installed on CFBboilers. Roller mill systems utilize hot air (mill air) for drying oflimestone and transportation to the CFB. Mill air is typically obtainedfrom the primary air stream as the mills utilize air at elevatedpressures. For most CFB boilers, hot primary air in the range of about400° F. to about 600° F. is typically available. The hot air is used todry the entering limestone, sweep the limestone out of the mill, andconvey it to the boiler. However, in order to protect the mechanicalcomponents of roller mills, the operating temperatures thereof are keptrelatively low, for example in the range of about 170° F. to about 200°F.

Furthermore, limestone generally contains low quantities of moisture(e.g., less than 5%). For proper system operation, the air temperatureentering the roller mills is a function of the acceptable millmechanical operating temperature, the limestone moisture content, andthe ratio of air to limestone. For a JIT system as described above, airtemperatures entering the mill will generally be less than about 250° F.Therefore, given the available primary air temperature in the range ofabout 500° F., the mill air obtained from the primary air stream istherefore tempered in order to produce a mill exit air temperature,typically in the range of about 180° F. to 225° F.

Experience has shown that the use of tempering air effectively reducesheat recovery from the flue gas. In turn, the reduced heat recoverylowers boiler efficiency, as an example, in the range of about 0.5% toabout 1.5%, thereby increasing equipment size and operating costs.Although the installation of a JIT limestone system using roller millson a CFB boiler provides an initial benefit with respect to reducedcapital cost for the CFB boiler, there is a tradeoff with respect to anincrease in long term operating costs (e.g., as high as 15% to 30%compared to the cost of the CFB boiler). Accordingly, it would bedesirable to be able to improve heat recovery and thus increase boilerefficiency for a CFB boiler equipped with a JIT limestone system.

SUMMARY

According to aspects illustrated herein, a sorbent conditioning and feedapparatus for a circulating fluidized bed combustion system includes adryer apparatus configured to dry unprocessed sorbent materialtransported thereto from a raw storage container; a sorbent crushingdevice configured to reduce the particle size of dried sorbent materialdischarged from the dryer apparatus; and a circulating fluidized bed(CFB) boiler configured to receive processed sorbent material conveyedfrom the sorbent crushing device; wherein the dryer apparatus utilizesan untempered hot air source diverted directly from a primary air streaminput to the circulating fluidized bed boiler.

According to other aspects illustrated herein, a direct limestoneconditioning and feed apparatus for a circulating fluidized bedcombustion system includes a dryer apparatus configured to dryunprocessed limestone material transported thereto from a raw storagecontainer; a roll crushing device configured to reduce the particle sizeof dried limestone material discharged from the dryer apparatus; and acirculating fluidized bed (CFB) boiler configured to receive processedlimestone material conveyed from the roll crushing device; wherein thedryer apparatus utilizes an untempered hot air source diverted directlyfrom a primary air stream input to the circulating fluidized bed boiler.

According to other aspects illustrated herein, a method of conditioningand feeding sorbent material for a circulating fluidized bed combustionsystem includes transporting raw, unprocessed sorbent material from araw storage container to a dryer apparatus; diverting untempered hot airdirectly from a primary air stream input to the circulating fluidizedbed boiler, the untempered hot air used to dry the raw sorbent material;directing dried raw sorbent material from the dryer apparatus to acrushing device configured to reduce the particle size of the dried rawsorbent material discharged from the dryer apparatus; and conveyingprocessed sorbent material from the crushing device to the CFB boiler.

The above described and other features are exemplified by the followingfigures and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the figures, which are exemplary embodiments, andwherein the like elements are numbered alike:

FIG. 1 is a schematic block diagram of an existing sorbentpreparation/feed apparatus, including a roller mill which utilizestempered air to dry and separate sorbent material;

FIG. 2 is a schematic block diagram of an improved direct sorbentpreparation/feed apparatus and method for a CFB boiler system, utilizinguntempered air for sorbent drying, in accordance with an embodiment ofthe invention; and

FIG. 3 is a schematic diagram of a CFB boiler system incorporating animproved direct sorbent preparation/feed apparatus, utilizing untemperedair for sorbent drying, in accordance with a further embodiment of theinvention.

DETAILED DESCRIPTION

Disclosed herein is an improved direct sorbent preparation/feedapparatus and method for a CFB system. Briefly stated, a sorbentpreparation/feed apparatus utilizes an alternate crushing device (i.e.,one that does not need the use of tempered air) with a separate, hightemperature dryer apparatus in order to supply dry, sized limestone on ajust-in-time basis to a CFB boiler without reducing boiler efficiency.

As indicated above, CFB boilers equipped with roller mill JIT limestonesystems employ tempering air in order to control the entering airtemperature, which results in reduced heat recovery from the flue gas.The reduced heat recovery in turn lowers boiler efficiency (e.g., on theorder of about 1%) and thus increases equipment size and operatingcosts. As will be illustrated from the embodiments discussed herein, aJIT system incorporating one or more roll crushers and utilizing aseparate limestone drying apparatus eliminates the use of tempering air,thus allowing for maximum heat recovery from a boiler air heater.

As used herein, “Primary Air” (PA), in the context of FBC boilers,refers to combustion air delivered to the bed fluidization grate at thebottom of the furnace. Primary air is generated at relatively highpressures. In addition, some primary air may be diverted to otherequipment, as well as enter the furnace at locations other than thegrate. Furthermore, in the context of FBC boilers, “Secondary Air” (SA)refers to combustion air delivered through openings in the furnacewalls. Secondary air is generated at pressures lower than primary air,and is used to fluidize the bed and stage combustion for emissioncontrol. “Tempered Air” refers to hot air that is cooled (i.e.,tempered) with relatively cold air, such as ambient air for example.“Untempered hot air” refers to hot air leaving an air heater of the FBCsystem. Both primary air and secondary air leaving the air heater areconsidered untempered hot air streams. “Unprocessed Sorbent” refers toraw sorbent delivered to the FBC boiler conditioning (size reduction &drying) system. Unprocessed sorbent has not been conditioned to theproper size and moisture content required for feeding to the FBC boiler.

Referring initially to FIG. 1, there is shown a schematic block diagramof an existing sorbent preparation/feed apparatus 100. As is shown, theapparatus 100 utilizes a roller mill 102 to both dry and separate theraw sorbent material. In order to control the temperature of the airutilized by the roller mill 102, hot primary air (PA) exiting the boilersystem air heater 104 is mixed with tempering air (e.g., from coldprimary air entering the air heater 104) so as to produce temperedprimary air for input to the roller mill 102. As further illustrated inFIG. 1, dampers 106 may be utilized in the hot primary air path and thetempering air path in order to produce the tempered air of a desiredtemperature and pressure. For purposes of illustration, FIG. 1 furtherillustrates additional inputs and outputs of the air heater 104,including cold input secondary air (SA), hot secondary air output to aCFB boiler (not shown), hot gas input to the heater 104 from a boilerbackpass heat exchanger 108, and exiting warm flue gas.

One reason for requiring tempered air in a conventional roller millpreparation/feed apparatus 100 relates to the temperature limits of thecrushing device (roller mill) itself. In addition, the velocity of theair conveying the conditioned limestone from the roller mill to the CFBboiler is controlled. If, for example, the velocity of the conveying airis too low, sorbent particles may settle out in the pipe, leading toblockage.

Accordingly, FIG. 2 is a schematic block diagram of an improved directsorbent preparation/feed apparatus 200 for a CFB boiler system,utilizing untempered air for sorbent drying, in accordance with anembodiment of the invention. In lieu of a roller mill, the apparatus 200utilizes a sorbent dryer 202 and separate roll crusher device 204. Thewet, raw sorbent is received into the sorbent dryer 202, wherein hot,untempered primary air (e.g., on the order of 400° F. or higher isdirectly input to the dryer 202 to dry the wet, raw sorbent.

In an exemplary embodiment, the sorbent dryer 202 is configured todischarge warm air, which carries sorbent fines (fine particles)directly to the boiler, thus bypassing the roll crusher device 204. Inthis sense, the sorbent dryer 202 has a particle separation capability.The remaining dry, raw sorbent that is not already of a fine particulatesize is input from the sorbent dryer 202 to the roll crusher 204. Assuch a device does not have the same temperature restrictions as aroller mill, the need to temper the air used to dry the particles iseliminated. Once the dry, raw sorbent particles are conditioned (i.e.,reduced to the desired size), they are conveyed (e.g., pneumatically ormechanically) to the boiler (not shown in FIG. 2) in accordance with adirect (JIT) feed system.

FIG. 3 is a schematic diagram of a CFB boiler system 300 incorporatingan improved direct sorbent preparation/feed apparatus (such as apparatus200 of FIG. 2, for example), utilizing untempered air for sorbentdrying, in accordance with a further embodiment of the invention. As isshown, the system 300 further includes a CFB boiler 302, a raw sorbent(e.g., limestone) storage facility/container 304 and cyclone 306. Raw,wet limestone is conveyed from the storage container 304, is metered andconveyed to the dryer 202. In an exemplary embodiment, limestonemetering may be controlled by sulfur reduction signals received from theassociated boiler control system. Again, hot, untempered air from theprimary air system is directly used as the source of heat to dry thelimestone, wherein only enough hot air is diverted from the primary airstream to the dryer 202 as may be needed to dry the limestone. The dryer202, having the capability of operating at high temperatures, istherefore is not subject to tempering.

As indicted above, the dryer 202 may include a particle separationcapability so as to remove sorbent fines with exiting warm air to theboiler, thereby preventing an unnecessary feeding of sorbent fines intothe roll crusher 204. One exemplary type of dryer in this regard may bea fluidized bed dryer. However, other types of high-temperature dryersmay also be used. In any event, warm air containing evaporated water andlimestone dust (sorbent fines) is conveyed directly to the boiler 302 aspart of the combustion air. On the other hand, limestone discharged fromthe dryer 202 is conveyed to one or more roll type crushers 204 arrangedin series where it is crushed to a desired size and discharged.Downstream of the roll crushers 204, the dried, sized limestone isconveyed (e.g., pneumatically or mechanically) to the CFB boiler 302.Although not specifically depicted in FIG. 3, the system 300 may alsoinclude surge hoppers and metering systems downstream of the rollcrusher 304 to enhance system operation and flexibility.

Through the use of the above described CFB boiler system and improveddirect sorbent preparation/feed apparatus embodiments, the total amountof pressurized primary air in the system may be reduced. Because theelimination of tempering air reduces the amount of primary air needed todry the sorbent, the total primary air is reduced and replaced by lowerpressure secondary air.

Moreover, the above exemplary embodiments further improve particle sizedistribution of the conditioned limestone to the CFB boiler. Asdiscussed, limestone (sorbent) to the boiler requires a specific rangeof sizes for optimum sulfur capture. If the size distribution becomescoarser, the quantity of unused sorbent rises. Larger particles are notsufficiently broken down in the furnace before removal, thus resultingin a greater amount of unused sorbent. On the other hand, if the sizedistribution becomes too fine, the quantity of unused sorbent rises dueto reduced residence time for the sulfur capture reactions.

Accordingly, improved particle size distribution is achieved when smallparticles are removed (classified) prior to entering the roll crusher.If allowed to enter the crusher, many initially small particles are madeeven smaller, thus increasing the amount of unused sorbent. By using aseparate sorbent dryer with direct contact of hot air (e.g., a counterflow flash dryer), entrainment of the smaller particles can be achievedthus avoiding additional size reduction in the crusher.

Further, it will be appreciated that the disclosed embodiments reducethe amount of high pressure air (primary and tempering) in the CFBsystem. This reduction is obtained primarily by using higher temperatureair to dry the sorbent. The reduction lowers total power consumption ofthe air fans, which in turn improves boiler economics. For example, theuse of primary air at a maximum air heater outlet temperature maximizesenergy recovery from the flue gas, thus increasing boiler efficiencywith respect to a JIT system using tempering air. As the boilerefficiency is significantly increased, a long-term savings in plantoperating costs is realized. By way of example, the boiler efficiencymay be increased in the range of about 0.5% to about 1.5%.

While the invention has been described with reference to variousexemplary embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A sorbent conditioning and feed apparatus for a circulating fluidizedbed (CFB) combustion system, comprising: a dryer apparatus configured todry unprocessed sorbent material transported thereto from a raw storagecontainer; a sorbent crushing device configured to reduce the particlesize of dried sorbent material discharged from the dryer apparatus; anda circulating fluidized bed boiler configured to receive processedsorbent material conveyed from the sorbent crushing device; wherein thedryer apparatus utilizes an untempered hot air source diverted directlyfrom a primary air stream input to the circulating fluidized bed boiler.2. The apparatus of claim 1, wherein the temperature of the untemperedhot air source is at least about 400° F.
 3. The apparatus of claim 1,wherein the dryer apparatus is further configured to remove sorbentfines from the unprocessed sorbent material prior to input to thesorbent crushing device.
 4. The apparatus of claim 3, wherein thesorbent fines removed by the dryer apparatus are diverted directly tothe CFB boiler.
 5. The apparatus of claim 3, wherein the sorbent finesremoved by the dryer apparatus are diverted to the CFB boiler using aportion of boiler system secondary air discharged from the dryerapparatus.
 6. The apparatus of claim 1, wherein the sorbent crushingdevice comprises a roll crusher.
 7. A direct limestone conditioning andfeed apparatus for a circulating fluidized bed (CFB) combustion system,comprising: a dryer apparatus configured to dry unprocessed limestonematerial transported thereto from a raw storage container; a rollcrushing device configured to reduce the particle size of driedlimestone material discharged from the dryer apparatus; and acirculating fluidized bed boiler configured to receive processedlimestone material conveyed from the roll crushing device; wherein thedryer apparatus utilizes an untempered hot air source diverted directlyfrom a primary air stream input to the circulating fluidized bed boiler.8. The apparatus of claim 7, wherein the temperature of the untemperedhot air source is at least about 400° F.
 9. The apparatus of claim 7,wherein the dryer apparatus is further configured to remove limestonefines from the unprocessed limestone material prior to input to the rollcrushing device.
 10. The apparatus of claim 9, wherein the limestonefines removed by the dryer apparatus are diverted directly to the CFBboiler.
 11. The apparatus of claim 9, wherein the limestone finesremoved by the dryer apparatus are diverted to the CFB boiler using aportion of boiler system secondary air discharged from the dryerapparatus.
 12. The apparatus of claim 7, wherein the dryer apparatuscomprises a fluidized bed dryer.
 13. A method of conditioning andfeeding sorbent material for a circulating fluidized bed combustionsystem, the method comprising: transporting raw, unprocessed sorbentmaterial from a raw storage container to a dryer apparatus; divertinguntempered hot air directly from a primary air stream input to thecirculating fluidized bed boiler, the untempered hot air used to dry theraw sorbent material; directing dried raw sorbent material from thedryer apparatus to a crushing device configured to reduce the particlesize of the dried raw sorbent material discharged from the dryerapparatus; and conveying processed sorbent material from the crushingdevice to the CFB boiler.
 14. The method of claim 13, wherein thetemperature of the untempered hot air source is at least about 400° F.15. The method of claim 13, further comprising using the dryer apparatusto remove sorbent fines from the raw sorbent material prior to directingthe dried raw sorbent material from the dryer apparatus to a crushingdevice.
 16. The method of claim 15, wherein the sorbent fines removed bythe dryer apparatus are diverted directly to the CFB boiler.
 17. Themethod of claim 15, wherein the sorbent fines removed by the dryerapparatus are diverted to the CFB boiler using a portion of boilersystem secondary air discharged from the dryer apparatus.
 18. The methodof claim 13, wherein the dryer apparatus comprises a fluidized beddryer.
 19. The method of claim 13, wherein the sorbent crushing devicecomprises a roll crusher.
 20. The method of claim 13, wherein thesorbent comprises limestone.